1. Field of the Invention
The present invention relates to a method of processing fiber pulp used to make paper products, and, more particularly, to a method of removing bleedthroughs from fiber pulp used to make paper products.
2. Description of the Related Art
The use of old corrugated container (OCC) for making new containerboard increased rapidly in the 1980s and early 1990s. According to American Forest and Paper Association, the recycling rate of OCC, defined as the ratio of the tonnage of OCC grade fiber used at paper and paperboard mills to that of total containerboard supply, increased from 39.5% in 1987 to 63.4% in 1996. Since then, the recycling rate has stabilized around 63.5%.
At such high recycling rates, removal of contaminants from OCC becomes crucial to assure quality standards of the product made from OCC fiber. Currently, the standard process to recycle OCC consists of a pulper, detrasher, high-density cleaner, coarse screen, forward cleaner, fine screen, reverse or through-flow cleaner and thickener. Though the majority of contaminants can be removed by such a processing sequence, many fine contaminant particles remain in the processed pulp. These fine particles may either be brought in by the contaminated OCC or generated by the fragmentation of larger particles during the treatment. Tests on some fine screen accepts indicate that the size of these fine contaminants ranges from 80 xcexcm to 750 xcexcm in equivalent diameter, with an average of about 170 xcexcm. Therefore, the majority of them can easily pass through 0.20 mm (200 xcexcm) slotted screen baskets, which is used in most OCC recycle mill fine screen systems. Many large particles are also able to pass through the screen baskets due to their shape or flexibility. Reverse and through-flow cleaners are also proven to be ineffective to remove these contaminants since the specific density of xe2x80x9cbleed-throughsxe2x80x9d are close to 1 and their size is too small to create enough drag forces to differentiate them from fibers.
The failure of removing these fine contaminants, together with the increasing usage of OCC, has caused deterioration of pulp qualities. The greatest concern is the formation of objectionable xe2x80x9cbleed-throughsxe2x80x9d in dried paperboard. These xe2x80x9cbleed-throughtsxe2x80x9d are formed through melting of fine contaminants. At the elevated temperature (approximately 350xc2x0 F.) of a paper machine dryer section, most of these contaminants will melt and migrate into voids of the fiber web. At the spot where the contaminant occupies, a dark bleed-through is formed and it gives the finished paperboard an objectionable appearance. In peak times, the concentration of xe2x80x9cbleed-throughsxe2x80x9d in paper sheets, tested with an image analyzer, reaches 50,000 ppm, that is, 5% of total sheet area is covered with dark xe2x80x9cbleed-throughsxe2x80x9d. The xe2x80x9cbleed-throughsxe2x80x9d could also cause problems in converting operations, such as brown tissue embossing and gypsum-board making.
The worst situation occurs in mills using 100% recycled fiber. In these mills, each time the OCC is recycled, it brings in more contaminants, they will accumulate over years until the pulp becomes finally unsuitable for making new paper products.
What is needed in the art is a method of effectively removing bleed-through contaminants form OCC fiber pulp.
The present invention provides a system for removing bleed-through contaminants from OCC fiber pulp, including a plurality of series arranged primary flotation cells with respective reject outlets which are combined in parallel and coupled with one or more secondary flotation cells and a water clarifier.
The invention comprises, in one form thereof, a system for removing bleed-through contaminants from OCC fiber pulp. A plurality of primary flotation cells are provided with each primary flotation cell having an inlet, an accept outlet and a reject outlet. The inlet and the accept outlets of the plurality of primary flotation cells are coupled together in a series configuration. The reject outlets are coupled together in parallel. A secondary flotation cell has an inlet, at least one accept outlet and a reject outlet. The secondary flotation cell inlet is fluidly coupled with the reject outlet of the primary flotation cells. A water clarifier has an inlet, a clarified water outlet and a sludge outlet. The inlet is connected with the reject outlet of the upstream secondary flotation cell.
An advantage of the present invention is that fine contaminants causing bleed-through flaws in a fiber web are removed from the OCC fiber pulp.
Another advantage is that fibers from the contaminant laden rejects discharged from the primary flotation cells are further processed within one or more secondary flotation cells to remove the fibers for use.